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FIGURE3.2 (a) The relationship between Ed5o and the DOA and (b) the relationship between Ed5o and Ad5o for all 204 intra-storm measurements (b), (c) the relationship between Ed5o and the DOA, and (d) the relationship between Ed50 and Ad50 based on the measurements made on the River Exe at Thorverton.

FIGURE3.2 (a) The relationship between Ed5o and the DOA and (b) the relationship between Ed5o and Ad5o for all 204 intra-storm measurements (b), (c) the relationship between Ed5o and the DOA, and (d) the relationship between Ed50 and Ad50 based on the measurements made on the River Exe at Thorverton.

intra-storm temporal scales, respectively. These aspects of EPSD behavior at the study sites are considered in detail below.

3.4.2 Short-Term Variability in the EPSD

Short-term (<1 min) variations in the effective particle size characteristics of suspended sediment were assessed by analysis of the replicate measurements of the EPSD made during each periodic deployment of the Par-Tec equipment. During each periodic deployment of ca. 3 min, replicate measurements were made at intervals of

25.6 sec. For each set of replicate measurements, the temporal variability of the EPSD was defined by the standard error (S.E.) of three key descriptive statistics, namely, the mean value of the volume mean and median particle size, as well as the degree of sorting as defined by Buller and McManus32 which describes the spread of the particle size data.

Mean variation in any of the three parameters did not exceed ±0.3 xm at any site, and in the majority of cases the maximum variation on any occasion, in any parameter, did not exceed ±2.0 xm. Maximum variation in the three parameters was shown by the S.E. of the median particle size at ±2.2 ¡m at the R. Culm and Exe study sites on the May 25, 1994, and this value principally reflects the significantly coarser EPSD of suspended sediment on this date, relative to that measured during all other storm events at the eight study sites. In view of the limited variation evident in the replicate EPSD data, and taking account of the considerably greater degree of intra- and inter-storm event variation in EPSD discussed below, the EPSD of suspended sediment at the study sites can be considered to be stable over periods of ca. 3 min. Accordingly, in the following analyses the EPSD of suspended sediment transported during each periodic ca. 3 min deployment of the Par-Tec equipment has been characterised by averaging the associated replicate measurements.

3.4.3 Intra-Storm Variability in the EPSD and Associated DOA

The most detailed intra-storm measurements of the EPSD were made at Jackmoor Brook on the April 8, 1994, and the River Exe at Thorverton on February 23, 1994. On these occasions the Par-Tec equipment was deployed for extended periods through the storm event, during which time in situ measurements were made every 25.6 sec and averaged over intervals of ca. 3 min. Figure 3.3(a) and (b) present the semi-continuous median effective particle size data for these two sites, together with the median APSD of periodic samples taken during the events. Figure 3.3(c) and (d) present the associated hydrograph and suspended sediment concentration data for the Jackmoor Brook and River Exe storm events, respectively.

The EPSD data for the Jackmoor Brook event encompasses both the rising and falling limb of the hydrograph and a range in suspended sediment concentration of ca. 800 mgl-1, with a maximum value of 1040 mgl-1. The data for the River Exe event are confined to the peak and falling limb of the hydrograph and suspended sediment concentrations are generally less than at the Jackmoor Brook, with a range of ca. 220 mg l-1 and a maximum value of 358 mg l-1. At both sites, the Ed50 data exhibit a gradual reduction in magnitude through the storm event, the pattern of which approximates an exponential trend. At both sites it is apparent from the Ad50 data that the Ed50 reflects aggregation throughout the sampling period, and, given the limited range in the Ad50 data, that the magnitude of this phenomenon follows a similar trend to that of the Ed50, with the DOA being greatest during the earlier parts of the hydrograph, and in particular, declining rapidly through the falling limb.

Table 3.2 summarizes the intra-storm range in Ed50 and associated DOA for all storm events at the eight study sites. From these data it is clear that in every case Ed50 exhibits measurable variation at the intra-storm temporal scale. The magnitude of this

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